PSUBSCRIBE for pattern matching in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What messages does this PSUBSCRIBE receive?

Given the Redis command PSUBSCRIBE news.*, which published messages will the subscriber receive?

Redis

PSUBSCRIBE news.*

PUBLISH news.sports "Sports update"
PUBLISH news.weather "Weather update"
PUBLISH news "General news"
PUBLISH news.sports.football "Football news"

A Receives messages from channels: news only

B Receives messages from channels: news, news.sports, news.weather

C Receives messages from channels: news, news.sports.football

D Receives messages from channels: news.sports, news.weather, news.sports.football

💡 Hint

Remember that the asterisk (*) matches any characters in the channel name after the prefix.

The pattern 'news.*' matches any channel starting with 'news.' followed by any characters. So it matches 'news.sports', 'news.weather', and 'news.sports.football'. It does not match 'news' alone because there is no dot after 'news'.

📝 Syntax

intermediate

Identify the correct PSUBSCRIBE command syntax

Which of the following Redis commands correctly subscribes to all channels starting with 'chat.'?

A PSUBSCRIBE chat.*

B SUBSCRIBE chat.*

C PSUBSCRIBE chat

D SUBSCRIBE chat

💡 Hint

PSUBSCRIBE supports patterns, SUBSCRIBE does not.

PSUBSCRIBE is used for pattern matching subscriptions. The pattern 'chat.*' matches all channels starting with 'chat.'. SUBSCRIBE does not support patterns and treats the argument as a literal channel name.

❓ optimization

advanced

Optimizing pattern subscriptions for multiple similar channels

You want to subscribe to all channels related to user notifications, which are named like 'user.123', 'user.456', 'user.789'. Which PSUBSCRIBE pattern is the most efficient and correct?

A PSUBSCRIBE user.?

B PSUBSCRIBE user.*

C PSUBSCRIBE user.[0-9]*

D PSUBSCRIBE user

💡 Hint

Consider how Redis pattern matching works with wildcards.

The pattern 'user.*' matches any channel starting with 'user.' followed by any characters, which covers all user notification channels. 'user.?' matches only channels with exactly one character after 'user.'. 'user.[0-9]*' requires the character immediately after 'user.' to be a digit followed by any characters, which is overly restrictive. 'user' matches only the exact channel 'user'.

🔧 Debug

advanced

Why does this PSUBSCRIBE pattern not receive expected messages?

A developer runs PSUBSCRIBE user.*.notifications but does not receive messages published to 'user.123notifications'. What is the likely cause?

A The PSUBSCRIBE command requires escaping dots, so the pattern should be 'user\. *.notifications'.

B Redis patterns treat '*' as matching only one character, so it does not match '123'.

C The pattern 'user.*.notifications' does not match 'user.123notifications' because Redis patterns do not support multiple wildcards separated by dots.

D The pattern is correct; the issue is that messages are not published to 'user.123notifications'.

💡 Hint

Check how Redis pattern matching treats the '*' wildcard.

In Redis, the '*' wildcard matches any sequence of characters including dots. However, the pattern 'user.*.notifications' expects a literal dot after the '*', so it matches channels starting with 'user.', followed by any characters (including dots), then a dot, then 'notifications'. But since '*' can match dots, the pattern effectively expects two dots, which does not match 'user.123notifications' as expected. The correct pattern to match 'user.123notifications' is 'user.*notifications' without the extra dot.

🧠 Conceptual

expert

Understanding PSUBSCRIBE pattern matching behavior

Which statement correctly describes how Redis PSUBSCRIBE pattern matching works?

A The '*' wildcard matches any sequence of characters, including dots, and patterns are matched against the entire channel name.

B The '*' wildcard matches only characters up to the first dot, so patterns cannot match across dots.

C Patterns in PSUBSCRIBE are matched only against the prefix of channel names, not the entire name.

D PSUBSCRIBE patterns require regular expressions syntax for matching channel names.

💡 Hint

Think about how the '*' wildcard behaves in Redis channel patterns.

In Redis PSUBSCRIBE, the '*' wildcard matches any sequence of characters, including dots. Patterns are matched against the entire channel name, not just a prefix. Redis does not use regular expressions for PSUBSCRIBE patterns.